1. Field of the Invention
The present invention relates to a radio base station, a relay station, and a relay method using radio communication. The present invention is especially advantageous when it is applied to a case where a radio communication system prescribed by IEEE 802.16 is used as a base system and thereto a relay station is added.
2. Description of the Related Art
A radio communication system carrying out communication with the use of a radio channel, i.e., WCDMA, CDMA2000 or such, as a typical example, currently spreads worldwide. In such a radio communication system, a plurality of radio base stations are set for a service area, and each radio terminal carries out communication with another communication apparatus (i.e., another communication terminal) with the use of any of the radio base stations. At this time, an overlapping area is provided with an adjacent service area in which an adjacent base station can carry out radio communication, and, handover to the adjacent radio base station is available when a radio environment degrades.
Further, as a radio communication method, for example, code division multiplexing, time division multiplexing, frequency division multiplexing, OFDMA or such, is applicable. In these methods, generally speaking, a plurality of radio terminals can connect to a single radio base station simultaneously.
FIG. 1 illustrates a radio communication system with a single radio base station (BS) and plural radio terminals (MS) belonging under the radio base station. Radio communication is performed based on a so-called P-MP connection, in which plural radio terminals can be connected to a single radio base station.
However, even within the service area in which the radio base station can carry out radio communication, high speed communication may not be available in a place close to the area boundary, since the radio environment may not be satisfactory. Further, even within the area, radio signal propagation may be obstructed by a cause such as building shading. Thus, an area (so-called dead zone) in which satisfactory radio connection with the radio base station is difficult may occur.
In order to solve the problem, a plan in which a relay station is disposed in the service area of the radio bases station, and radio communication is available between the radio terminal and the radio base station with the use of the relay station, has been proposed.
Especially, in a task group of 802.16j, introduction of such a relay station (RS) is currently being studied.
FIG. 2 illustrates a radio communication system including a relay station.
The radio base station transmits a preamble, MAP data, and burst data. A radio terminal (MS #1) directly performs radio communication with the radio base station, and a radio terminal (MS #2) performs radio communication with the radio base station via a relay station (RS).
The above-described items concerning IEEE 802.16 are disclosed in the following non-patent documents 1 and 2.
Non-patent document 1: IEEE Std 802.16TM-2004
Non-patent document 2: IEEE Std 802.16eTM-2005
In general radio communication systems, reception errors may be caused by conditions of transmission channels. In such cases, the transmitting side apparatus resends signals to the receiving side apparatus (e.g., ARQ (Automatic Repeat reQuest), HARQ (Hybrid Automatic Repeat reQuest)), so that the transmission data is transmitted to the receiving side apparatus.
However, if the above-described relay station is installed in the radio communication system, the radio terminal will be able to perform radio communications with the radio base station via the relay station, but the following problem will arise. That is, the radio segment will be divided into plural segments by the relay station (in this case, into two segments), and therefore, additional measures will be necessary for controlling the resending operation in an efficient manner.